Optical isolators are used in a variety of applications in optical communication systems. Generally, optical isolators are used to prevent reflective portions of transmitted signals from re-entering the transmitting device. Many older prior art designs prevent reflections from re-entering a transmitting device in a polarization-selective manner. However, in certain circumstances where a transmission system causes uncontrollable changes in polarization, the polarization state of a signal may be unknown, and thus, these earlier polarization dependent designs are not considered to be practical. Thus, as of late, a large effort has been undertaken to develop an isolator that is polarization independent. It is also desired to have an optical isolator that is capable of isolating high power optical signals without compromising the longevity of the isolator.
One prior art polarization independent optical isolator is described in U.S. Pat. No. 5,033,830 issued Jul. 23, 1991 in the name of Jameson and entitled Polarization Independent Optical Isolator. Jameson describes an isolator having a single birefringent plate, a pair of stacked reciprocal rotators, a Faraday rotator, and a reflector positioned in tandem adjacent to the birefringent plate. In a forward (transmitting) direction, a lightwave signal exiting an optical fiber is split into a pair of orthogonal rays by the birefringent plate. The orthogonal rays then pass through a first reciprocal rotator and the Faraday rotator which provides 22.5.degree. of rotation. The rotated rays are then redirected by the reflector back though the Faraday rotator. After passing through the second reciprocal rotator, the orthogonal rays re-enter the same birefringent plate where they are recombined and launched in an output fiber. Since a Faraday rotator is a non-reciprocal device, any signal traveling through the isolator in the reverse (isolation) direction will be split on both passes through the birefringent plate such that neither will intercept the input fiber. In practice, Jameson's single stage isolator described above, may provide adequate isolation; however, in some instances, increased isolation may be required. Such additional isolation has been known to be provided by using a multi-stage optical isolating device; generally these multi-stage devices are costly to manufacture often requiring nearly double the number of optical components that a single stage device requires; more importantly, aligning nearly twice as many components with one another can be difficult, time-consuming, costly, and generally increase the overall alignment error.
For example, U.S. Pat. No. 5,581,640 in the name Pan et al. Assigned to E-tek Dynamics, Inc. describes a multi-stage optical isolator wherein two polarizers in the form of a birefringent crystal wedges of lithium niobate are used as the birefringent material of the polarizers. The polarizers in prior art FIG. 1 (shown as FIG. 6A of the '640 patent) are shown as spaced-apart crystal wedges having complementary slanted faces. However, it is not clear from the specification whether the space between the optical elements, shown between all of the elements in the device, is merely for the purpose of illustration. For example, if the gapped end faces of elements 64a and 64b in FIG. 1 are air gapped, then the end faces would likely require anti-reflection (AR) coating.
The instant invention obviates both the requirement of AR coating two crystals disposed between two non-reciprocal polarization rotating elements, and, obviates using two such crystals, thereby obviating the requirement for adhesive between such crystals.
Since the instant invention obviates the requirement of two crystals disposed between two non-reciprocal polarization rotating elements, it thereby obviates the complex and difficult component alignment that is required when using two such crystals are in tandem in a multi-stage isolator.
It is an object of this invention to provide a relatively low-cost optical isolator that is particularly well suited to carrying high-power optical signals.